President Barack Obama joined European leaders in their struggle to deflect a dagger aimed at the heart of the continent's political unity.

It's the combined impact of home-grown terrorism, mass migration from the Middle East and North Africa, sluggish economic growth, the lingering effect of the sovereign debt crisis and a groundswell of nationalist sentiment.

European unity "is under strain," Obama said at a press conference with British Prime Minister David Cameron on Friday. He offered hope that "the ties that bind Europe together are ultimately much stronger than the forces that are trying to pull it apart.

British Prime Minister David Cameron and President Barack Obama wave to well-wishers outside No. 10 Downing St. in London on Friday. Bloomberg photo by Simon Dawson

The news conference came on the first full day of a visit to the continent that also will include a summit in Germany on Monday with leaders of Europe's four largest economies. He came directly from a conference in the Middle East with Saudi King Salman and leaders of other Persian Gulf Arab states, a meeting at which the Syrian civil war and regional instability driving migration to Europe figured prominently.

Economic and cultural ties between the nations of Europe haven't been so strained since the Maastricht Treaty created the European Union in 1993. An influx of more than a million migrants last year has given new life to xenophobic political parties. Terrorist attacks in Paris and Brussels have fueled concern about the continent's lenient border controls and internal security.

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The British government has warned that a vote to leave the union would grievously damage both the British and European economies.

"It's the confluence of these crises that is so striking," said Karen Donfried, a senior Obama adviser on Europe until 2014 who's now president of the German Marshall Fund of the United States.

A fracturing of the 28-nation EU or a descent into dysfunction would undermine growth prospects for an economic bloc that is America's largest trading partner and would threaten cohesion among the U.S.'s bedrock allies. In the worst case, divisions could unravel a project of integration that, since its inception as a six-nation coal and steel trade bloc in the aftermath of World War II, has maintained a rare period of extended peace among the major nations of a continent once racked by warfare.

Obama published an opinion article in the Daily Telegraph newspaper shortly after landing in London on Thursday that urged British voters to reject a so-called Brexit. He amplified his message in the news conference, pointing to the relative peace and economic prosperity

Europe has enjoyed since World War II.

Europe in the 21st century "looks an awful lot better" than in the 20th century, he said in the news conference. "I think a majority of Europeans recognize that." The Brexit vote "will send a signal that is relevant about whether the kind of prosperity that we've built together is going to continue."

He also issued a veiled economic threat, warning Britons to be wary of claims made by Brexit backers that the United States would quickly reach a trade agreement with Britain to maintain preferences the country receives as part of the European Union.

"Maybe at some point down the line there might be a U.K.-U.S. trade agreement, but it's not going to happen anytime soon," Obama said.

"The U.K. is going to be in the back of the queue."

Obama arrived in London well aware of the delicacy of a foreign leader taking sides on matters that are essentially internal European issues.

London Mayor Boris Johnson, who favors leaving the EU, told the Associated Press "it's paradoxical that the United States, which wouldn't dream of allowing the slightest infringement of its own sovereignty, should be lecturing other countries about the need to enmesh themselves ever deeper in a federal super-state."

Obama approached the subject gingerly, saying he didn't come to "fix the vote" but address a subject in which the U.S. has a "deep interest."

"Part of our special relationship, part of being friends, is to be honest and to let you know what I think," Obama said.

Support for European unity has been a foundation of U.S. global strategy under both Democratic and Republican administrations for decades.

"If you start seeing divisions in Europe, that weakens NATO," Obama said. "That'll have an impact on our collective security."

He will likely revisit the subject at a town hall for young people on Saturday, as well as in a speech in Germany on Monday.

As a presidential candidate, Obama attracted his largest crowd of the campaign at an outdoor rally in Berlin in 2008, with estimates that as many as 200,000 people attended. He remains a popular figure in Europe. Seventy-six percent of Britons and 73 percent of Germans said they had confidence in him to do the right thing in world affairs, according to Pew Research Center polls published last year. Only 58 percent of Americans agreed.

Obama is even more popular among Europe's youth, and the president's schedule in Britain, including the event for young people, is "very clearly geared at driving youth turnout in the referendum because young people are more pro-European," said Jacob Kirkegaard, a senior fellow at the Peterson Institute for International Economics in Washington.

While Cameron's strategy to defeat Brexit emphasizes warnings of economic peril, that provides an opening for Obama to lay out a positive vision for Britain's future in the European Union, Kirkegaard said.

In Germany, Obama's other stop, Chancellor Angela Merkel, the continent's dominant political figure, has been weakened by the flood of refugees into her country. Obama will offer his appreciation for Merkel's "bold leadership in responding to the refugee crisis," said Charles Kupchan, the National Security Council's senior director for European affairs. "The president wants to provide political support to her for doing so."

More than a million migrants entered Germany last year. The influx is credited for losses by Merkel's party in three state elections last month, as well as the rise of Alternative for Germany, a populist-right party whose platform calls for closing the nation's borders.

Even some Germans who support resettling refugees are dismayed by Merkel's handling of the crisis, particularly her concessions to Turkish President Recep Tayyip Erdogan in a bid to slow the flood of migrants. In addition to a 6 billion euro ($6.7 billion) aid package offered by the EU, Merkel granted Turkey's request to prosecute Jan Boehmermann, a German satirist who lampooned Erdogan in a bawdy poem.

Members of Merkel's governing caucus said the decision was anti-democratic.

Merkel back-tracked on Friday. While sticking with the decision to open a criminal case against the comedian, she apologized for comments she and her press secretary made criticizing the poem because they may have suggested that press freedom "is no longer important."

"I'm angry at myself that I spoke of 'deliberately hurtful' on April 4 because that created the impression that my personal opinion has anything to do with this," Merkel said at a news conference in Berlin. "In retrospect, that was a mistake."

Obama will confer on Monday in Hanover with Merkel, Cameron, French President Francois Hollande and Italian Prime Minister Matteo Renzi regarding the Syrian civil war and strife in Iraq, Libya and Afghanistan that is fueling migration to Europe, as well as counterterrorism programs and a strategy to counter Russian President Vladimir Putin's aggressive moves in Eastern Europe.

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